Rocket Speed & Altitude

[Todd Burch]

Sorry for the geektastic question but I googled this for a few minutes and didn’t find the answer so thought I’d ask the SpaceX geeks here. I’m a mechanical engineer with a decent physics background so I speak the jive, but I had a few questions:

SpaceX webcasts always show the speed and altitude of the rocket.

1. Are these values “predicted” based on known quantities (thrust, mass, etc) and rocket equations, or are they actually measured values?

2. If they are measured, how are they measured? Do they use GPS or something else?

3. If they are measured, relativity comes into play. What is the reference for 0 velocity and 0 altitude? Since it’s zero at launch it seems that both are relative to the launch pad, but that would mean that as the earth rotates, the velocity of the launch pad itself changes and would affect the apparent velocity of the rocket (for example).

Anyone know how this is done?

 

[ecarfan]

Good questions, but I think only a SpaceX employee with specific knowledge of those issues could answer them with a high degree of confidence.

My speculative answers would be:

1 - Measured values

2 - GPS?

3 - Relativistic effects are negligible at the speeds in question

 

[RDoc]

The relative velocity question is interesting (at least to me). Since the launch pad is on the surface of a rotating sphere, it's moving in 3 space, but the rocket is as well, and they are not rotating in the same planes. More importantly, when the rocket first launched, it already had the radial velocity of the launch point due to the Earth's rotation. That definitely is important, but not explicitly called out.

What counts for orbiting is the angular velocity and radial distance relative to the Earth's center of gravity. However, that's not what is shown. My guess is that the reported velocity and altitude are relative to the point on the Earth's surface directly under the rocket with the velocity being the magnitude of the vector sum of the 3 axes' vectors. As the second stage pitches forward to thrust nearly parallel to the surface, the velocity and altitude shown are closer to what you want.

Perhaps they should report another number, fraction of orbital speed at the current location.